Assessment of Design Methods in Existing PBSD Standards Project

Summary:

The project continues the assessment of the applicability and accuracy of adopting first-generation Performance Based Seismic Engineering (PBSE) analysis methods that were developed for evaluating existing buildings for use in performance-based design of new buildings, specifically focused on buckling-restrained braced frames (BFBFs). The project will evaluate the performance of code-compliant steel BRBF buildings using the first generation PBSE techniques now used by practitioners. The evaluations will be used to validate current PBSE methods or identify needed changes to them that will result in advancing PBSE, and to evaluate design approaches and performance where buckling-restrained brace BRB systems are employed. This project is a follow-on to earlier in-house investigations on the application of PBSE in designing special steel moment and braced frames and will use archetypical building systems that were designed during that work.

Description:

Objective: The objective of this study is to continue supporting refinement of first-generation Performance-Based Seismic Engineering (PBSE) analysis methods, specifically focused on buckling-restrained braced frame (BRBF) buildings. This will be accomplished by evaulating the seismic performances of several steel BRBF buildings that will be designed in accordance with ASCE/SEI 7-10[1] (referenced hereafter as ASCE 7) requirements for new buildings and then assessed and redesigned using PBSE methods promulgated in ASCE/SEI 41-06[2] (referenced hereafter as ASCE 41) for existing buildings that are being used by practitioners to implement PBSE. The study will be concluded by the end of FY 2014.

What is the new technical idea? With no specific "PBSE technique" in ASCE 7, practitioners who design new buildings using performance-based methodologies often apply the techniques developed for evaluating existing buildings that are detailed in ASCE 41 to analyze and design new buildings – such approaches are typically described as "first-generation" PBSE analysis techniques. However, potential problems arise with this approach because the developers of ASCE 41 adopted many conservative assumptions to address the large uncertainties in evaluating existing (often older) buildings; there has been no comprehensive study done to correlate the two standards. The performance of BRBFs was identified in the BSSC Roadmap[3] document as a priority topic for study. They have become widely used in higher seismicity areas – their popularity has increased dramatically since their acceptance as valid braced frame systems in the ANSI/AISC 341-05[4] specification and the ASCE/SEI 7-05[5] standard. Over 450 buildings in the U.S. have been constructed using BRBFs[6]. Much of this rise in popularity is due to the ability of designers to "tune" the capacities of a brace to nearly the exact demands required by design. ASCE 41-13 is soon to be released, and BRBFs are included for the first time in its assessment criteria tables.

In the earlier phase of work under this project that is being completed in FY 2013, eighteen steel moment frame and braced frame buildings were designed and analyzed using ASCE 7. ASCE 41-based PBSE techniques were then applied to these same buildings, with the goal being to assess how well the first-generation PBSE approaches to design align with the approach found in ASCE 7. Results from the earlier phase indicate that the conservatism of the ASCE 41 evaluation approach is inconsistent with the prescriptive procedures of ASCE 7.

For the proposed new work, the performance of several BRBFs designed using ASCE 7 will be assessed and potentially redesigned using ASCE 41-based approaches. The assessment will identify inconsistencies between the two methodologies, with the results used to suggest improvements needed for PBSE methods based on ASCE 41 and, potentially, improvements needed for the prescriptive procedures of ASCE 7.

What is the research plan? A research framework similar to that already used to assess 18 other steel lateral force resisting systems (including steel special moment frames, concentrically brace frames, and eccentrically braced frames) will be applied to a suite of six BRBF buildings. The BRBF systems will be based on adapting the archetypical building designs already developed in the previous PBSE-ASCE 41 steel frame project. The BRBFs will be designed in accordance with ASCE 7 and AISC 341-10[7] prescriptive provisions. Each BRBF system will be designed for two target seismic hazard levels representing regions of high seismicity; modeled in detail, including material and geometric nonlinearities; and analyzed using nonlinear finite element analysis programs to determine dynamic response for a suite of selected earthquake ground motions. Each BRBF system will then be subjected to rigorious evaluation using all four of ASCE 41's assessement procedures, with particular emphasis given to nonlinear dynamic analysis, to judge compliance with predefined seismic performance objectives. Based on the ASCE 41 assessment and comparison with the ASCE 7 design results, the applicability of using ASCE 41 PBSE for designing new BRBF structures will be quantified, indicating under what conditions the ASCE 41 methodology does or does not meet the design intent of the ASCE 7 standard. The design and assessment results will be vetted with a small, highly qualified external peer review panel to ensure high fidelity and maximum impact. The final assessment will be the first of its kind, and will be disseminated through NIST technical notes and peer-reviewed journal articles.

[2] Seismic Rehabilitation of Existing Buildings, ASCE 41-06, American Society of Civil Engineers, 2007. It is anticipated that the 2013 edition of this document may be released during the time period of this work, in which case it will be referenced.

Major Accomplishments:

Outcome:

Results from the draft final report for the PBSE-ASCE 41 project and the detailed peer review meetings indicate that the NIST approach to this design and analysis project is accurate and consistent. Peer reveiwers agree that there are inconsistencies between the seismic performance for new buildings required by ASCE 7 and the performance required by ASCE 41. Steel frame buildings designed in accordance ASCE 7 for areas of high seismicity can perform well. However, the same buildings analyzed using PBSE ASCE 41-based methods are not deemed to perform at levels consistent with ASCE 7 in many cases. The results indicate that ASCE 41 provides a more conservative design result than is the case for ASCE 7. The inconsistencies between the two standards is important, because many practitioners are applying ASCE 41 to conduct detailed PBSE design of new buildings, resulting in more conservative, and thus more costly, design outcomes than required by ASCE 7. Application to the BRBF question will extend this work to address another increasingly used system.